In the flyback converter mode, energy is stored in the transformer during the phase in which the switching transistor is switched on, and this is subsequently transmitted to the secondary side, during the phase in which the switching transistor is switched off. In this case, however, disturbing voltage spikes are produced when the switching transisitor switches off, and these lead to a severe voltage load on the switching transistor, as well as causing radiated interference. It is thus known for a damping network, also referred to as a snubber network, to be provided in parallel with the primary winding, and this network is used to limit the voltage spikes of the current input of the switching transistor when the switching transistor switches off. This network normally has a capcitor in parallel with the primary winding, which capcitor is charged when the switching transistor is switched off, and whose energy is then emitted to an energy-storage capacitor on the input side, or is consumed in a resistor and converted into heat. A damping network of this type is known, by way of example, from DE-A-40 29 221.
EP-A-0 695 023 discloses a switched-mode power supply which, in addition to a damping network in parallel with the primary winding, also has a second damping network in parallel with a winding on the secondary side of the transformer. In this case, a series circuit formed by a capcaitor and a second diode is connected in parallel with a rectifying diode, which produces a desired output voltage across a charge-storage capacitor, and the centre point of this series circuit is connected to a reference potential via a diode and an inductance. Energy is in this case likewise transferred to the charge-storage capacitor via this capacitor, for energy recovery purposes, when the switching transistor switches off.
The method of operation of this switch-mode power supply will now be described in more detail with reference to FIG. 1. This switched-mode power supply has a network connection UN, to which a network rectifier 1 followed by an energy-storage capacitor C0 are connected, across which a DC voltage U0 is produced. This is applied to a primary winding WP of a transformer TR, in series with which a switching transistor T1 is connected. The switching transistor T1 is driven in a known manner by a driver circuit 2, illustrated only symbolically here, with switching pulses 3. The switched-mode power supply in this case operates in particular in the flyback converter mode, in which energy is transferred from the transformer TR to one or more secondary windings during the phase in which the switching transitor T1 is switched off.
A damping network for damping voltage spikes when the transistor T1 is switched off is connected in parallel with the primary winding WP. This damping network has a series circuit formed by a capacitor Cs and a diode Ds, with a resistor Rs being connected in parallel with its diode. The voltage spikes which are produced when the switching transistor T1 switches off are in this case absorbed by the capacitor Cs and are partially fed back via the diode Ds to the energy-storage capacitor C0, while the remainder is converted into heat in the resistor Rs.
On the secondary side, the switched-mode power supply has a secondary winding W1, which provides a desired output voltage U1 across a rectifier diode D1 and a charge-storage capacitor C1. In order to suppress voltage spikes and in order to reduce the power losses in the switched-mode power supply, a damping network is likewise coupled to the secondary winding W1. This damping network has a first series circuit with a capacitor C2 and a diode D2 in parallel with the diode D1, as well as a second series circuit with an inductance L1 and a diode D4, which is arranged between a tap a on the first series circuit and a reference potential. A resistor R1 is also connected in parallel with the inductance L1 and is used to damp oscillations between this inductance and the capacitor C2.
The network on the secondary side, as shown in FIG. 1, operates as follows: during the phase in which the switching transistor T1 is switched on, the voltage Us on the secondary winding W1 is negative, so that the capacitor C2 is charged via the inductance L1 and the diode D4. Thus, in the process, energy is stored in the capacitor C2, with the inductance L1 limiting the current rise during the charging process. When the switching transistor T1 is switched off, the voltage Us rises rapidly, until, in the end, the voltage U1 is reached, and the diode D1 changes to the forward-biased state, with the charge-storage capacitor C1 thus once again being recharged. However, the output voltage U1 is reached at an earlier stage across the capacitor C2, since the centre point a of the series circuit formed by C2 and D2 is positive with respect to the voltage Us. The diode D2 thus becomes forward-biased before the diode D1, so that the voltage rise of the voltage Us is damped. The energy in the capacitor C2 is likewise transferred to the charge-storage capacitor C1 during this process. In this case, any oscillations which occur between the capacitor C2 and the inductance L1 are damped by the resistor R1. Further details of the operation of this damping network are described in EP-A-0 695 023, which is hereby referred to.
A further damping network with a damping network on the primary side and a damping network on the secondary side is known from EP-A-0 279 335.